The present invention relates to a method for punching rotor iron core pieces for use in a laminated rotor core of an electric motor or stator core pieces for use in a laminated stator core of an electric motor.
For example, Japanese Laid-Open Patent Publication No. 2004-153928 discloses such a method for punching. In accordance with this method, a belt-like original material sheet 91 is fed in the longitudinal direction, and stator core pieces 92 are consecutively punched from the original material sheet 91 as shown in FIGS. 20 and 21. A plurality of projections 93 are formed on the outer circumference of each stator core piece 92. Each projection 93 has an attachment hole 94 for receiving a bolt. A plurality of slits 95 for receiving coils are formed on the inner circumference of the stator core 92. The slits 95 are formed at predetermined angular intervals. A magnetic pole portion 96 is formed between each pair of adjacent slits 95.
The thus punched stator core pieces 92 are laminated onto and fixed to each other, so that a laminated stator core for an electric motor is formed. In this case, variations in the flatness and thickness of the original material sheet 91 can adversely affect the lamination state of the laminated stator core. To prevent this, a rotated lamination method has been employed. In accordance with the rotated lamination method, punched stator core pieces 92 are laminated while the center angle of each stator core piece 92 is shifted in relation to other stator core pieces 92, so as to cancel out variations in flatness.
The method for punching stator core pieces 92 according to Japanese Laid-Open Patent Publication No. 2004-153928 will now described in detail. At a station prior to the station shown in FIG. 20, the original material sheet 91 is punched to form slits 95 and magnetic pole portions 96 on the inner circumference of a stator core piece 92. Subsequently, at the station shown in FIG. 20, the original material sheet 91 is punched to form openings 97 and attachment holes 94 for receiving bolts in the outer periphery of the stator core piece 92. The openings 97 are substantially U-shaped in a plan view so as to form the projections 93. Next, at a station shown in FIG. 21, the original material sheet 91 is punched to form arcuate portions 98, which define the outer circumference of the stator core piece 92. Each arcuate portion 98 is formed to connect proximal ends of an adjacent pair of the openings 97.
As described above, according to the method disclosed in the publication, the openings 97 for forming the projections 93 and the arcuate portions 98 for defining the outer circumference of the stator core piece 92 are formed through punching at different stations, while the original material sheet 91 is being fed. Therefore, the punching positions on the original material sheet 91 are likely to be displaced between the stations. Specifically, each arcuate portion 98 might fail to connect the proximal ends of the corresponding adjacent pair of the openings 97. In this case, since a coupling portion such as a micro-joint is formed between the arcuate portion 98 and the opening 97, the stator core piece 92 cannot be separated from the original material sheet 91. Even if the stator core piece 92 is separated from the original material sheet 91, a blur remains as traces of the micro-joint. This can hinder subsequent processes.
To prevent such drawbacks, the proximal ends of the openings 97 need to be extended further radially inward than the outer circumference of the stator core piece 92 as shown in FIG. 22. However, in this case, a recess 93a is formed at each proximal end of the projections 93 at the outer circumference of the stator core piece 92. Excessive stress thus tends to be concentrated on each recess 93a. This is undesirable in view of the strength. Further, since each projection 93 is formed by the U-shaped opening 97, the proximal portion (base portion) of the projection 93 is likely to be bent. This can adversely affect the assembly accuracy of the rotated lamination of the stator core pieces 92.